To compare the theories of stellar evolution and the observed properties, One of the most powerful tools is Hertzsprung-Russel, or H-R, diagram. It is the diagram first plotted by the Danish astronomer Ejnar Hertzsprung and the American astronomer Henry Norris Russel, in which the luminosities of the stars are plotted against their surface temperatures.
In this diagram each star is represented by a dot. The vertical axis represents the star’s luminosity. Luminosity is the amount of energy a star radiates in one second. The horizontal axis represents the stars surface temperature and is usually labelled using the Kelvin temperature scale.
It can be seen that most of the stars lie along a continuous sequence which runs from the bottom right to the top left of the diagram. This sequence is known as main sequence and the stars belonging to it, known as main sequence stars, are all in steady state of burning hydrogen into helium. Along the main sequence the mass of the stars are shown and it is obvious that they increase along it from bottom right to top left. Sun is a very average star. The stars on the main sequence have masses between one tenth and 60 times the mass of the sun but their luminosities range from about 100,000 times the luminosity of the sun for the most massive stars to only about one thousandth for the lowest mass stars. There are good physical reasons for this. The temperatures shown in the horizontal axis refer to the surfaces of the stars but their central temperatures are very much greater than these values. Now, the low mass stars towards the bottom right of the main sequence have smaller surface temperature than the sun, therefore their central temperature must be low as well. If the mass of the star is too small, the central temperature will be too low to sustain nuclear reaction to convert hydrogen into Helium. These stars are unstable. Furthermore, stars with masses greater than about 50 to 100 times the mass of the sun are also unstable. These are enormously luminous stars, about 100,000 times luminous than our sun. In these stars, most of the internal pressure is provided by the pressure of radiation rather than by the pressure of hot gas. If this pressure becomes too great, the outer layers of the star are blown off and this creates instability. These very massive stars are so luminous that they burn up their nuclear fuel very rapidly so their life time is very short. Thus, these are excellent reasons why all the stars we observe in the universe should have masses within the range of the sun.
